JP 2002-206442 A discloses an air-fuel ratio control device for an internal combustion engine. An engine which is controlled by the air-fuel ratio control device comprises an exhaust gas sensor, an exhaust gas purifying catalyst and a fuel injector. The air-fuel ratio control device performs a feedback control based on an output of the exhaust gas when the engine is operated in a first operation region. In the feedback control, a drive period of the fuel injector is controlled so that the output of the exhaust gas closes to a stoichiometry point.
The air-fuel ratio control device performs a rich operation control when the engine is operated in a second operation region. The second region is an operation region located on a higher load side or a higher engine speed side than the first region. When the engine is operated in such a high region, bed temperature of the exhaust gas purifying catalyst tend to rise easily. The rich operation control, the air-fuel ration is enriched based on an open-loop control. When the rich operation control is performed, temperature of the exhaust gas decreases due to latent heat of vaporization of excess HS. Therefore, it is possible to suppress an excessive increase in the bed temperature when the engine is operated in the second region.
The air-fuel ratio control device also continues to perform the feedback control for a while when the operating condition of the engine enters the second operation region from the first operation region. The duration of the feedback control is increased or decreased according to the duration of operation in the first operation region before entering the second operation region. The longer the operation duration, the easier the bed temperature rise after entering the second operation region. By increasing or decreasing the duration of the feedback control depending on the duration, it is possible to ensure safety against the excessive rise in the bed temperature and to keep the fuel consumption low.
The above mentioned control after entering the second operation region is translated to a temporarily control in which the first operation region is temporarily enlarged or reduced. Such a region variable control is also be expected to apply to the second operation region. This is because the longer the operation duration in the second operation region is, the easier it is for the bed temperature to rise when the operating condition enters the first operation region from the second operation region. When the region variable control is applied to the second operation region, then the second operation region is temporarily enlarged or reduced.
The present inventor is examining control based on a closing timing of an intake valve selected according to an operating condition of an engine, an ignition period of an ignition device, and EGR rate. The engine to be controlled by the control under consideration comprises two types of intake cams for driving the intake valve, a supercharged EGR system, a three-way catalyst as an exhaust gas purifying catalyst, and an ignition device.
The two types of the intake cams include a large cam whose working angle and lift amount are relatively large and a small cam whose working angle and lift amount are relatively small. The large cam is configured to close the intake valve in a first crank angle section. The first crank angle includes a crank angle at which inhalation efficiency becomes maximum under a predetermined condition. The small cam is configured to close the intake valve in a second crank angle section which is located on an advance side relative to the first crank angle section. The second crank angle includes a crank angle at which the inhalation efficiency under the predetermined condition is relatively low.
The supercharged EGR system is a system, so-called a LPL-EGR system, comprising a supercharger including an exhaust turbine and an intake compressor, and an EGR device that introduces exhaust gas on the downstream side of the three-way catalyst into the upstream side of the intake compressor.
In the control under consideration, the large cam is selected in a high EGR operation region where a target value of EGR rate (hereinafter also referred to as “target EGR rate”) is set to a predetermined high value. Further, in the control under consideration, the small cam is selected in a low EGR operation region where the target EGR rate is set to a value lower than the predetermined high value. Furthermore, in the control under consideration, when the small cam is selected, an ignition period is advanced as compared with a case where the large cam is selected. According to such engine control, it is possible to improve engine output in both the high EGR operation region and the low EGR operation region.
However, the following problem is developed when the region variable control described above is performed in addition to the engine control under consideration. That is, if these two controls are performed in parallel, a rich operation region temporarily enlarged by the region variable control may overlap with the high EGR operation region under consideration. Then, excess HC is discharged from the cylinder in the overlapping region. Despite the excess HC, the three-way catalyst is not able to demonstrate its original purification capability because the engine is driven in the rich operation region. Also, in the high EGR operation region, the exhaust gas which passes through the three-way catalyst is recirculated to an intake system of the engine as external EGR gas. Therefore, during the overlapped region, deposits derived from the excess HC tend to occur in the intake system.
The present disclosure addresses the above described problem, and an object of the present disclosure is to take measure in the LPL-EGR system when the rich operation region is overlapped with the high EGR operation region.
A first aspect of the present disclosure is a control device for an internal combustion engine.
The control device is configured to control an engine.
The engine comprising:
two types of intake cams for driving an intake valve;
a turbocharger including an exhaust turbine and an intake compressor;
an exhaust gas purifying catalyst which is provided on a downstream of the exhaust turbine;
an EGR device which is configured to introduce exhaust gas on the downstream side of the exhaust gas purifying catalyst into an upstream side of the intake compressor as an external EGR gas; and
an ignition device which is configured to ignite air-fuel mixture in a cylinder.
The control device is configured to, based on an operation condition of the engine which is specified by engine torque and engine speed, set a target EGR rate and a target air-fuel ratio and select from the intake cams a drive cam for the intake valve.
The control device is further configured to, when the engine is operated in a high EGR operation region where the target EGR rate is set to a predetermined high EGR rate:
select a first cam as the drive cam; and
set a closing timing of the intake valve to a first crank angle section including a crank angle at which inhalation efficiency is the highest under a condition where engine speed and supercharging pressure are fixed,
The control device is further configured to, when the engine is operated in a low EGR operation region where is located on a higher torque and higher engine speed side relative to the high EGR operation region and the target EGR rate is set to a lower rate than the predetermined high EGR rate:
select a second cam as the drive cam which has smaller operation angle and smaller lift amount than those of the first cam;
set the closing timing of the intake valve to a second crank angle section which is located on an advanced side relative to the first crank angle section and whose inhalation efficiency is lower than that during the first crank angle section; and
change an ignition period of the ignition device to an advance period relative to the ignition period which is set when the engine is operated in the high EGR operation region.
The control device is further configured to, when the engine is operated in an operation region where the high EGR operation region is overlapped with a rich operation region where the target air-fuel is set to a rich value:
change the target EGR rate to a lower value than the predetermined high EGR rate;
select the second cam as the drive cam;
set the closing timing to the second crank angle section; and
change the ignition period to the advance period.
A second aspect of the present disclosure is the control device for the internal combustion engine according to the first aspect.
The control device comprising:
an EGR map in which the high EGR operation region and the low EGR operation region are associated with the operation condition of the engine;
an air-fuel ratio map in which the rich operation region and a stoichiometric operation region where the target air-fuel ration is set to a stoichiometric ratio are associated with the operation condition of the engine; and
a drive cam map in which a first cam operation region where the first cam is selected as the drive cam and a second cam operation region where the second cam is selected as the drive cam are associated with the operating condition of the engine.
The control device is further configured to:
determine, when the EGR map is superimposed on the air-fuel ratio map, whether or not the rich operation region overlaps with the high EGR operation region; and
change, when it is determined that the rich operation region overlaps with the high EGR operation region, a cam boundary between the first cam operation region and the second cam operation region so that all of the rich operation region fall within the second cam operation region.
A third aspect of the present disclosure is the control device for the internal combustion engine according to the first aspect or the second aspect.
The engine further comprising a waste gate valve which is provided on a bypass pipe of the exhaust turbine.
The control device further comprising opening degree maps in which an opening degree of the waste gate valve is associated with the opening condition of the engine, wherein the opening degree maps are set for the first cam and the second cam, each of the opening degree maps includes full close lines for specifying the waste gate valve to be fully closed.
The control device is further configured to change, when it is determined that the rich operation region overlaps with the high EGR operation region, the cam boundary so that all of the rich operation region fall within the second cam operation region and also so that the cam boundary locates at a lower engine speed side relative to the full close line of the opening degree map for the second cam.
According to the first aspect, when the engine is operated in the high EGR operation region, the first cam is selected as the drive cam for the intake valve and the intake valve is closed at the first crank angle section. When the engine is operated in the high EGR operation region, the target EGR rate is set to the predetermined high rate. Therefore, in this case, a knocking limit is relatively high. Further, the first crank angle section includes the crank angle at which inhalation efficiency is the highest under the condition where engine speed and supercharging pressure are fixed. Therefore, when the first cam is selected and the intake valve is closed at the first crank angle section, it is possible to improve engine output.
Further, according to the first aspect, when the engine is operated in the low EGR operation region, the second cam is selected as the drive cam for the intake valve, the intake valve is closed at the second crank angle section and the air-fuel mixture is ignited at the advance period relative to the ignition period which is set when the engine is operated in the high EGR operation region. When the engine is operated in the low EGR operation region, the target EGR rate is set to a lower value than the case where the engine is operated in the high EGR operation region. That means the knocking limit decrease in the EGR operation region. In this respect, the second cam has smaller operation angle and smaller lift amount than those of the first cam. And the second crank angle section is located on the advanced side relative to the first crank angle section and whose inhalation efficiency is lower than that during the first crank angle section. Therefore, when the second cam is selected and the intake valve is closed at the second crank angle section, it is possible to lower the inhalation efficiency and suppress the decrease in the knocking limit. Further, when the air-fuel mixture is ignited at the advance period relative to the ignition period which is set when the engine is operated in the high EGR operation region, it is possible to compensate the decrease in the inhalation efficiency and suppress the decrease in the engine output.
Furthermore, according to the first aspect, when the engine is operated in the operation region where the high EGR operation region is overlapped with the rich operation region, the target EGR rate is changed to the lower value than the predetermined high EGR rate, the second cam is selected as the drive cam for the intake valve, the intake valve is closed at the second crank section and the air-fuel mixture is ignited at the advance period relative to the ignition period which is set when the engine is operated in the high EGR operation region. When the target EGR rate is changed to the lower value than the predetermined high EGR rate, amount of the external EGR gas is decreased. Therefore, in this case, it is possible to suppress the occurrence of deposit derived from excess HC. On the other hand, however, the knocking limit is lowered at the same time. In this respect, when the second cam is selected and the intake valve is closed at the second crank angle section, it is possible to lower the inhalation efficiency and suppress the decrease in the knocking limit. Further, when the air-fuel mixture is ignited at the advance period relative to the ignition period which is set when the engine is operated in the high EGR operation region, it is possible to compensate the decrease in the inhalation efficiency and suppress the decrease in the engine output.
According to the second aspect, when it is determined that the rich operation region overlaps with the high EGR operation region, it is possible to change the cam boundary so that all of the rich operation region fall within the second cam operation region. Therefore, it is possible to select the second cam as the drive cam for the intake valve whenever the engine is operated in the overlapped region.
According to the third aspect, it is possible to not only change the cam boundary so that all of the rich operation region fall within the second cam operation region but also change the cam boundary so that it locates at a lower engine speed side relative to the full close line of the opening degree map for the second cam. Therefore, it is possible to avoid the second cam being selected as the drive cam for the intake valve in the operation region of the lower engine speed side relative to the full close line of the opening degree map for the second cam.